Question: In triangle $ABC$, point $D$ is on segment $BC$, the measure of angle $BAC$ is 40 degrees, and triangle $ABD$ is a reflection of triangle $ACD$ over segment $AD$. What is the measure of angle $B$? [asy] size(101); pair B = (0,0); pair C = (2,0); pair D = (1,0); pair A = (1, tan(70*pi/180)); draw(A--B--C--cycle,linewidth(1)); label("$A$",A,N); label("$B$",B,W); label("$C$",C,E); draw(A--D,linetype("2 4")+linewidth(1)); label("$D$",D,S);[/asy]
Solution: Since $\triangle ADB$ is the mirror image of $\triangle ADC$, we have that $m\angle B = m\angle C$. Since $\triangle ABC$ is a triangle, we have that $m\angle A + m\angle B + m\angle C = 180^\circ$. Solving, we find that $m\angle B = \frac{180^\circ - 40^\circ}{2} = \boxed{70^\circ}$.